In general, since a solid electrolyte type fuel cell can use hydrogen gas, natural gas, methanol, coal gas or the like as a fuel, it can promote the substitution of alternative energy for oil in electric power generation, and further, since waste heat can be used, it has attracted attention from the viewpoint of resource savings and an environmental problem.
This solid electrolyte type fuel cell has a laminated structure as shown in an exploded perspective view of FIG. 1 and a sectional schematic view of FIG. 2. That is, a solid electrolyte type fuel cell 10 includes an electric power generation cell 14 made up of a solid electrolyte layer 11, a fuel electrode layer 12 and an air electrode layer 13, which are disposed at both sides of this solid electrolyte layer 11, a fuel electrode current collector 16 disposed to be in close contact with the fuel electrode layer 12, an air electrode current collector 18 disposed to be in close contact with the air electrode layer 13, and metal separators 17 constructed so that a fuel gas can be supplied to the fuel electrode layer 12 and an oxidizing agent gas containing oxygen can be supplied to the air electrode layer 13. Reference numerals 20 and 21 in FIG. 2 denote grooves which become a fuel passage and an air passage, respectively.
The conventional solid electrolyte type fuel cell is operated at a high temperature of 1000° C., so that chemical energy of fuel can be relatively efficiently converted into electric energy, however, in order to operate the solid electrolyte type fuel cell at 1000° C., materials used for component parts of the solid electrolyte type fuel cell are restricted especially to materials superior in heat resistance. For example, as a structure material of the separator or the like, it has been necessary to use dense ceramics such as lanthanum chromite (LaCrO3). Further, an attached apparatus (for example, a preheating apparatus of fuel gas, or the like) for operating the solid electrolyte type fuel cell is also required to be made of a material resistant to high temperature, and because of the operation at a high temperature, the consumption of the material becomes quick, the use life becomes short, and it is inevitable that the cost becomes high. Thus, in recent years, a solid electrolyte type fuel cell which can be efficiently operated at a temperature lower than 1000° C. and in which metal material can be used for peripheral members, has been developed.
In such a low temperature operating solid electrolyte type fuel cell, lanthanum gallate oxide, Sc-added zirconium, Y-added zirconium, ceria base oxide or the like is used for a solid electrolyte layer. By using these materials, the operation temperature can be lowered to about 700° C., and a metal material can be used for a peripheral member such as a separator. As a metal separator material, stainless steel, nickel base heat resistant alloy, cobalt base alloy or the like is used.
Besides, an air electrode current collector material is one of a plurality of important members influencing the power generation performance of a fuel cell, and mesh-shaped platinum is conventionally used as the air electrode current collector material.
However, the surface of the conventional metal separator material is covered with a chromium oxide film under conditions of, for example, 700° C. in the air, and the chromium oxide has conductivity at a high temperature, and has such a property that when temperature is lowered, its electric resistance increases. Accordingly, in the case where the operation is performed at a low temperature of about 700° C., it has a defect that the electric resistance is too large to be used as a separator material. Thus, even in the case where the fuel cell is operated at a low temperature while metal material is used for the separator, a material having a smaller electric resistance has been required.
Besides, in the case where a platinum mesh is used as an air electrode current collector, platinum is expensive since it is a noble metal, and for reduction in cost, a high performance air electrode current collector material substituting for platinum has been desired.